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Leaning Tower of Pisa
The construction began in 1173 and it must have been suspended at the termination of the third ring, around ten years later, since a subsidence of the soil of between 30 and 40 cm. had thrown the tower out of the perpendicular, causing an initial overhang of circa 5 cm. More than a century after the laying of the foundation stone, was once again began (1275) by Giovanni di Simone, who added three more levels, correcting the axis of the Campanile. In 1284 the six stories of loggias were to all effects finished, bringing the height of the building to 48 m., and employing a technical expedient that was meant to diminish, at least optically, the effects of the inclination, accomplished by raising the galleries of the upper floors on that side.
At the time the inclination of the Tower was more than 90 cm. The tormented problems of the Tower did not, as one might expect, greatly worry those who were involved in the construction and completion. The long intervals between building activities were dictated, most likely, by the need of letting the Campanile 'rest', but above all by letting both the foundations and the ground on which they rested settle down.
In a certain sense it can be said that the subsidence of the soil and the resultant inclination had, on the whole, been predicted. At the beginning of the 14th century the bells were placed at the sixth level, in the large opening still visible in the marble cylinder beyond the loggia. Between 1350 and 1372 Tommaso di Andrea Pisano (according to Vasari) terminated the installation of the belfry on the summit of the sixth order of loggias, increasing the correction of the axis, and thus diminishing the load on the side that was in inclination, which in the mean while had become fixed at 1.43 m.
Considered not only as a bell tower, but also as a belvedere for the square below - from the earliest times the loggias have served as "grandstand" for religious facts and fairs - it rises 58.36 m above the level of the foundation, just under 56 m over the level of the countryside, and its inclination, measured at the base, is over 4 m. The average subsidence of the base is 2.25 m, while the progression of the overhang, despite all attempts so far made to bring it to a halt, is about 1.2 mm per year.
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